Managing the foamy slag layer is one of the trickier aspects of electric-arc furnace (EF) steelmaking. Achieving an optimal foaming slag layer--which is a mixture of liquid, solid and gas particles formed by injecting slag with carbon monoxide--is critical to promoting low thermal conductivity, removing undesirable elements from the molten steel, shielding the electric arcs and protecting the furnaces fireproof lining.
It is very hard to determine whether you have too much or too little foaming slag, what the quality of the slag is and whether it is distributed uniformly in the (EF), said Thomas Matschullat, head of metals technologies at Munich-based Siemens AG. A lot of energy can be lost when too much carbon is injected into a slag and the foaming slag layer is too high, he said. On the other hand, if there isnt enough carbon and the foaming slag layer is too thin, it might not be adequate to protect the furnace wall from the high temperatures of the furnaces arc.
Siemens has developed a fully automated foaming slag management process based on structure-borne sound to take the guesswork out of maintaining the optimal foaming slag layer in the EF. In commercial use since 2008, Siemens Simelt FSM (foaming slag manager) is already being used by two European steelmakers and is going through final testing before being formally implemented by a major U.S. EF steelmaker.
Matschullat said EF steelmakers currently rely on the instincts of a good furnace operator, one who has a good feel of the steelmaking process, to determine whether the foamy slag layer is optimum. These technicians typically use predefined operating diagrams or manually inject carbon fines and oxygen but are not able to directly observe how these manual injections are influencing the height and distribution of the foaming slag.
He said that while these technicians are adept at measuring furnace state variables and interpreting external cues, they might not be able to determine whether there is too much or too little foaming slag in any particular region of the furnace or determine the quality of the slag layer throughout the furnace, especially given the noisy conditions inside the furnace.
Traditional methods of controlling foaming slag rely on a combination of static programs and operator intuition, Siemens said, and while technicians are skilled, they control the foaming slag process using imprecise modeling techniques which represent the conditions in the furnace. While workers undoubtedly strive to do their best, inaccuracies in the automated process, along with misjudgments, contribute to higher energy costs, higher injection carbon usage, accelerated electrode wear and potentially premature wear to refractory walls, the company said in a recent white paper.
It has been a long road in trying to find a more accurate solution. Siemens noted that in the late 1990s European researchers first looked to use online control sensors to improve slag foaming by mounting a sonic meter near a control room and recording the sound waves emitted by a nearby EF. They were able to determine some conditions within the furnace vessel and to use outputs from an inline control system to inject carbon and oxygen into the foaming slag layer, providing more precise and consistent control of the foamy slag height and distribution.
This control method, which was never commercialized, was a predecessor to the Siemens FSM technology, which uses rugged vibration sensors, or structural sound sensors, mounted on an adapter plate on the outside surface of the furnace opposite the electrode they are monitoring.
Matschullat said that while a slag layer cant be measured in millimeters, centimeters or even inches, the harmonics or vibrations generated by the electric arcs--which are transmitted through the steel, slag and gas phase--produce a signal that can be detected through the furnace wall. The FSM control module evaluates this structure-borne sound data and current signals using algorithms, which in turn regulate the injection of carbon and oxygen into different areas of the furnace to optimize slag conditions.
By measuring three current and vibration signals and evaluating the weight of the charged scrap along with the furnaces tilting angle and transformer taps, the control module is able to automatically regulate the injection of carbon and oxygen to precisely regulate the foaming slags height and distribution.
Visual displays provide two- and three-dimensional representations of what is occurring inside the sealed furnace wall in nearly real time, Siemens said. By calculating the dampening of the sound propagations, the controller determines the height of the foaming slag not just close to the electrodes but also in the complete area between the electrodes and the furnace shell.
Siemens said that a number of other industries have previously used vibration sensors to capture reliable, accurate and cost-effective data for advanced control systems, including railroads and other transportation, wind power, marine shipping, paper manufacturing and coal mining--industrial operations that frequently have loud, dangerous environments where complicated and expensive equipment must operate with minimal downtime.
One of the notable characteristics of vibration monitoring systems is that they are quite effective in these harsh conditions, Siemens said, explaining that vibration sensors typically do not have moving parts and are easily ruggedized to withstand extreme temperatures and other challenging environments.
Matschullat said that the railroad industry--both in the United States and Europe--has used data from vibration sensors to detect thermal cracks in wheel rims during offline inspections in order to prevent small problems from turning into major incidents. Union Pacific Railroad Co., for example, has supported an effort to develop a new technology for monitoring wheels. The vibration sensors monitor the wheels on passing trains for conditions that suggest fatigue or abnormal performance. Those sensors are tied to wireless devices and fiber-optic networks that relay the data to analytical software that can alert personnel, schedule maintenance, slow train speeds or even remove trains from service.
Siemens said that vibration monitors also have enabled wind farmers to keep an eye on critical components remotely rather than having skilled crews climb 300 feet to make routine maintenance checks. If telltale high- or low-frequency vibrations suggest a bearing or gearbox component is failing, they can acquire a component and undertake a repair before the issue causes damage or diminishes electricity production, the company said, noting that replacing a worn bearing on a wind turbine is a relatively simple $5,000 procedure while a failed bearing could result in a catastrophic gearbox or generator malfunction that could cost $245,000 to repair.
Matschullat said that Siemens began work on its FSM technology in 2005, once it realized that the technology used in its rail business was transferable to its metals technologies unit. It underwent testing in 2007 at a pilot installation at Meitingen, Germany-based Lech-Stahlwerke GmbH.
By having a better feel of the thickness of the foaming slag layer, we have been able to reduce our energy consumption by 10 to 15 kilowatt hours per tonne of crude steel we produce, and we have also saved 3 to 5 kilograms of coal per tonne of crude steel, said Hans Peter Markus, head of EF technology at Lech-Stahlwerke, which produces a total of about 1.4 million tonnes of steel per year from the two EFs. With a payback period of less than a year, the FSM technology also helped Lech-Stahlwerke meet governmental limits for carbon dioxide emissions.
The only fully commercialized installation of the technology at this time is at Byelorussian Steel Works (BMZ), the largest steelmaker in Belarus, according to Matschullat. BMZ is using it on its two EFs, which each produce about 800,000 tonnes of steel per year. Matschullat said that Zhlobin-based BMZ has seen savings at its furnaces comparable to those at Lech-Stahlwerke.
A major U.S. EF steel producer is in the middle of commissioning the technology at one of its several steelmaking facilities, with final testing of the technology expected to be completed in late spring. Matschullat acknowledged that Siemens had hoped to have more steel mills using this technology by now. We finished developing our product at a very bad time, given that in 2008 steelmakers didnt have a lot of money to make this kind of investment despite its many benefits, he said. This is a very conservative industry.
Siemens is in talks with a few steelmakers in Germany and Saudi Arabia, but Matschullat said that Siemens recent FSM sales efforts have been concentrated on North America, the largest market for electric furnace steelmaking. I believe there is potential for further developing this technology to not just use it to measure the foaming slag layer but for other indirect measurement in the steelmaking process, given the high temperatures and other adverse conditions in the furnace, Matschullat said, although he noted that would involve further development work.